The present invention is directed to semiconductor devices and, more particularly to a process for controlling and improving lifetime in a P-i-N diode.
P-i-N diodes are widely used for power circuit applications. Among the measures of diode performance are the following: rated forward current (IF), forward voltage drop (VF), transient forward recovery time (TFR), transient turn-on peak overshoot voltage (TOPO), diode reverse blocking, or breakdown voltage (BVR), transient reverse recovery time (TRR), and unclamped inductive switching (UIS), which is a measure of the amount of avalanche energy that can be dissipated in the device without destructive failure. TRR can be decreased by the introduction of recombination centers, but this is generally accompanied by an increase in VF.
In operation, a P-i-N diode is flooded with minority carriers during forward conduction, which results in very low resistivity of the undoped middle region, referred to as the xe2x80x9cintrinsicxe2x80x9d or xe2x80x9cixe2x80x9d region, during current flow and allows the diode to carry a high current density. The injection of a high concentration of minority carriers during forward conduction, however, also causes adverse effects. When the P-i-N diode is turned on with a high current flow, its initial forward voltage drop exceeds the voltage drop observed for the same current level at steady state conditions, an effect that is referred to as xe2x80x9cforward voltage overshoot.xe2x80x9d
A second, more serious drawback of a P-i-N diode has to do with its reverse recovery, as measured by TRR When the diode is switched from its on-state to its off-state, the minority carrier charge stored in the i-region of the diode during forward conduction must be removed by a large reverse current, followed by recombination. This reverse current surge is accompanied by a large voltage overshoot that results from flow of the reverse current through inductances in the circuit.
Increasing the switching frequency of a power circuit requires an increase in both turn-off speed and reverse current flow of the circuit components, leading to a degradation in system frequency. The operation and characteristics of P-i-N diodes are discussed in B. J. Baliga, Power Semiconductor Devices, 1996, PWS Publishing Company, Boston Mass., pages 153-182, the disclosure of which is incorporated herein by reference.
The current density of a P-i-N diode in its on-state is determined by recombination of minority carriers in its i region. Therefore faster reverse recovery, i.e., the process of switching the diode from its on- to its off-state, can be achieved through the introduction of recombination centers into its middle region by, for example, diffusion of gold or platinum, or by high energy electron irradiation. As discussed in the aforementioned work of Baliga at pages 55-59, the disclosure of which is incorporated herein by reference, the thermal diffusion of an impurity that exhibits deep levels in the energy gap of silicon, platinum, for example, is a commonly employed method for controlling lifetime in a power device.
U.S. Pat. No. 4,925,812, the disclosure of which is incorporated herein by reference, describes a process for introducing platinum atoms into a semiconductor body to reduce minority carrier lifetime that comprises the steps of forming layers first of palladium and then of platinum on a surface of the body, heating the body to form a layer of palladium silicide containing dissolved platinum atoms, and then heating the body to a temperature to allow platinum but not palladium atoms to diffuse into the body.
U.S. Pat. No. 5,283,202, the disclosure of which is incorporated herein by reference, describes a process for making a semiconductor power device with improved minority carrier lifetime control wherein a maximum dose of a transition metal, platinum or gold, is deposited on a surface of the device substrate adjacent a PN junction and metal atoms are diffused throughout the substrate in a gradient profile relative to the substrate surface.
Platinum diffuses into silicon much more rapidly than do atoms of boron or phosphorus, typical dopants for forming a diode. The diffusion of platinum into the device is carried out following its formation but prior to its metallization by, for example, aluminum. Platinum diffusion temperatures typically lie between about 850xc2x0 C. and 1000xc2x0 C.; at such temperatures, even small variations, as little as 5xc2x0 C., can produce large variations in the performance of a P-i-N diode. When the temperature of a substrate during platinum implantation diffusion is ramped up from, for example, 800xc2x0 C. to a target of 960xc2x0 C. over a period of about 30 minutes, a temperature overshoot of as much as 10xc2x0 C. can occur, requiring 10-15 minutes for the temperature to fall back to its intended level. Such an occurrence can have serious adverse consequences for the characteristics and performance of the diode. Furthermore, the process results, not in a desirable concentration of diffused platinum near the PN junction but in a substantially uniform platinum distribution throughout the substrate.
Thus, there continues to be a need for a method of reliably controlling and improving the lifetime characteristics of a P-i-N diode, including switching speed, forward voltage drop, and current leakage. The present invention meets this need.
The present invention is directed to an improved process for controlling and improving minority carrier lifetime in a P-i-N diode that comprises depositing platinum on a surface of a silicon semiconductor substrate containing at least one PN junction, heating the substrate to a temperature of about 800xc2x0 C., and diffusing the platinum into the substrate while increasing its temperature at a rate of about 5xc2x0 C./minute to a first selected temperature of about 850-950xc2x0 C. Platinum diffusion is continued while maintaining the substrate at the first selected temperature for about 30-60 minutes, then increasing the substrate temperature at a rate of about 5xc2x0 C./minute to a second selected temperature above 950xc2x0 C. to about 1000xc2x0 C., and maintaining the substrate at the second selected temperature for about 5-30 minutes before cooling.
The present invention is further directed to the formation of a P-i-N diode having improved minority carrier lifetime control.